In the semiconductor industry, a photolithography method using visible light or UV light has been employed as a technique of transcribing a fine pattern on a Si substrate or the like, which is required for forming an integrated circuit having such fine pattern. However, the microsizing of semiconductor devices has been accelerated, and on the other hand, the conventional photolithography method approaches the limit. In the photolithography method, it is said that the resolution limit for a pattern is about ½ of exposure wavelength and is about ¼ of exposure wavelength even if an immersion method is used. Even though the immersion method with an ArF laser (193 nm) is used, it is estimated that the limit is about 45 nm. Now, as an exposure technique for 45 nm or below, EUV lithography which is an exposure technique using EUV light having a further shorter wavelength than ArF laser has been considered to be promising. In this description, the EUV light indicates a ray having a wavelength in a soft X-ray region or a vacuum ultraviolet ray region, specifically, a ray having a wavelength of about 10 to 20 nm, particularly about 13.5 nm±0.3 nm.
The EUV light is apt to be absorbed by any substance and the refractive index of the substance at this wavelength is close to 1, and therefore, the conventional dioptric system such as one for photolithography using visible light or ultraviolet light cannot be employed. For this reason, a catoptric system, i.e. a reflective photomask (hereinafter referred to as “EUV mask”) and a mirror are employed in the EUV light lithography.
A mask blank is a laminated member before patterning which is used for producing a photomask. The EUV mask blank has a reflective layer for reflecting EUV light and an absorber layer for absorbing EUV light which are formed in this order on a substrate such as glass or the like (refer to Patent Document 1). As the absorber layer, a material having a high absorption coefficient of EUV light, specifically, a material having Ta as the main component, is used.
Patent Document 1 describes that a nitride of tantalum boron alloy (TaBN), an oxide of tantalum boron alloy (TaBO) and an oxynitride of tantalum boron alloy (TaBNO) are preferred as the material for the absorber layer since they have high absorption coefficient of EUV light and low reflectivity for deep UV light in the wavelength region (190 nm to 260 nm) of pattern inspection light.
In recent years, the thickness of the absorber layer of EUV mask blank is preferably thin. In EUV lithography, exposure light is not incident from a perpendicular direction to an EUV mask but incident from a direction at an angle of a few degrees, usually 6 degrees, to the perpendicular direction. If the thickness of the absorber layer is thick, at a time of EUV lithography, a shadow of the exposure light is formed on a mask pattern formed by removing a part of the absorber layer by etching, and the pattern accuracy or the dimension accuracy of a mask pattern (hereinafter referred to as “transcription pattern”) transcribed to a resist on a substrate such as a Si wafer by using the EUV mask, tends to be deteriorated. Since this problem becomes more significant as the line width of the mask pattern formed on the EUV mask becomes smaller, the thickness of the absorber layer of the EUV mask blank is required to be thinner.
For the absorber layer of the EUV mask blank, a material having a high absorption coefficient of EUV light is employed, and the thickness is ideally a thickness whereby EUV light incident into a surface of the absorber layer is completely absorbed. However, as described above, since the thickness of the absorber layer is required to be thin, it is not possible for the absorber layer to completely absorb EUV light incident into the layer, and a part of the incident light will be reflected from the layer.
The step of forming a transcription pattern of a resist on a substrate by EUV lithography requires the contrast of reflected light from the EUV mask, that is, the contrast between reflected light from a portion of the mask wherein the absorber layer is removed at a time of forming the mask pattern so that the reflective layer is exposed to the outside, and reflected light from a portion of the mask wherein the absorber layer is not removed at the time of forming the mask pattern. Accordingly, it has been considered that so long as a sufficient optical contrast of reflected light is obtained, there is no problem even if the incident EUV light is not completely absorbed by the absorber layer.
Based on the above concept, in order to reduce the thickness of the absorber layer, EUV masks using the principle of phase shift is proposed (refer to Patent Documents 2 and 3). They have characteristics that a portion from which the absorber layer is not removed at a time of forming a mask pattern has a reflectivity of from 5 to 15% for reflected light, and that reflected light from such a portion has a phase difference of 175 to 185° from reflected light from a portion where the absorber layer is removed at the time of forming the mask pattern so that the reflective layer is exposed to the outside. The documents describe that with the EUV masks, by using the principle of phase shift for reflected light from the absorber layer, it is possible to maintain a sufficient contrast to the reflective layer, and accordingly, it is possible to reduce the thickness of the absorber layer.